Semiconductor memory devices may be generally divided into volatile semiconductor memory devices, such as dynamic random access memory (DRAM) devices or static random access memory (SRAM) devices, and non-volatile semiconductor memory devices, such as flash memory devices or electrically erasable programmable read-only memory (EEPROM) devices. The volatile semiconductor memory devices generally lose data stored therein when power is turned off. However, non-volatile semiconductor memory devices may maintain stored data even when the power supply is interrupted or turned off.
Among the non-volatile semiconductor memory devices, the flash memory devices have been employed in various electronic apparatus, such as digital cameras, cellular phones, MP3 players, etc. Because a flash memory device generally requires a relatively significant time for writing or erasing data, alternative technologies for manufacturing semiconductor memory devices, for example, a magnetoresistive random access memory (MRAM) device, a ferroelectric random access memory (FRAM) device or a phase-change random access memory (PRAM) device, have been developed.
In a PRAM device, data may be input or output using a resistance difference between an amorphous state and a crystalline state of a phase-change material layer including a chalcogenide material, for example, germanium-antimony-tellurium (Ge—Sb—Te) (GST). Particularly, data having a value of “0” or “1” may be stored in the PRAM device using a reversible phase transition of the phase-change material layer. The phase-change material layer in the amorphous state may have a relatively increased resistance, whereas the phase-change material layer in the crystalline state may have a relatively decreased resistance. In the PRAM device, a transistor may provide the phase-change material layer with a reset current for changing the phase of the phase-change material layer from a crystalline state into an amorphous state. The transistor may also supply the phase-change material layer with a set current for changing the phase of the phase-change material layer from an amorphous state into a crystalline state. The phase-change material layer may be formed on a lower electrode making contact with a plug electrically connected to the transistor. Conventional PRAM devices and methods of manufacturing conventional PRAM devices are discussed in U.S. Pat. No. 5,825,046, U.S. Pat. No. 5,596,522, Korean Laid-Open Patent Publication No. 2005-31160 and Korean Patent No. 437458, for example.
In the methods of manufacturing conventional PRAM devices discussed above, a phase-change material layer including GST may be formed by a physical vapor deposition (PVD) process such as a sputtering process or a chemical vapor deposition (CVD) process. However, in the PVD process, controlling the composition ratio of GST may be difficult so that the phase-change material layer may not possess desirable electrical characteristics.
Particularly, GST is a pseudobinary compound, which includes germanium telluride (GeTe) and antimony telluride (Sb2Te3), having a chemical formula of germanium-antimony-tellurium (GexSbyTe(100-x-y)). Here, x and y may not have arbitrary values under 100, where GST has a composition ratio on a pseudobinary line that connects germanium telluride (GeTe) and antimony telluride (Sb2Te3) in a pseudobinary phase diagram, and the composition ratio may have only an acceptable error range of 10% (See IEE Proc.-Sci. Meas. Technol. 151, 394 (2004)).
FIG. 1 presents a ternary composition diagram of a phase-change material layer including GST formed by a composition-spread method. More specifically, germanium-antimony-tellurium (Ge2Sb2Te5) as used in a phase-change material layer may include about 22% by weight germanium (Ge), about 22% by weight antimony (Sb) and about 56% by weight tellurium (Te). When impurities, such as nitrogen (N), carbon (C), oxygen (O), silicon (Si), etc., are doped into the phase-change material layer, the phase-change material layer may have a changed composition ratio at least due to the amount of the impurities.
In a conventional sputtering method of forming a phase-change material layer, the composition ratio of the phase-change material layer may be changed by controlling the amount of germanium, antimony and tellurium. However, tellurium may have only a content in a range of about 50 to about 65% by weight in the phase-change material layer, at least because germanium telluride (GeTe) and antimony telluride (Sb2Te3) in the phase-change material layer commonly include tellurium. Reducing the content of tellurium to under about 50% by weight may be problematic.
Even when using a method allowing improved control of the composition ratio of a phase-change material layer as discussed in Korean Laid-Open Patent Publication No. 2006-599395, forming a phase-change material layer having a composition ratio in which tellurium has an amount outside the above-referenced range may present difficulties.